The present invention relates to equipment used in petroleum-production activities.
It relates to a separator for carrying out a process of gravitational separation of immiscible fluids of different densities.
More particularly, it relates to a piece of equipment for separating out the gaseous phase of a liquid/gas mixture for use, preferably, at the bottom of a petroleum well, so as to reduce the proportion of gas in the liquid to be pumped to allow the bottom pump to be able to operate more efficiently.
It may also be applied in the petrochemical, chemical or similar industries.
In nature, petroleum is generally mixed with water and gas.
When the flow pressure of a production well is low, one problem to be solved is that of deciding how to transfer the petroleum up from the bottom of the well to the site where it will undergo initial processing. Transfer may be by means of pumps of various types or of some other suitable artificial lift means, such as gas lift, for example. A decision of this type will depend, inter alia, on the characteristics of the fluids produced and on environmental conditions. By opting for pumping, the lift-system efficiency will be increased if the gaseous phase has already been separated from the liquid portion of the petroleum.
The object of the present invention is to promote efficient separation, even at the bottom of the well, of the gas which is mixed with the liquid phase of the petroleum so as to make viable the exploitation of certain onshore or offshore hydrocarbon reserves.
The separation of the fluid originating from the reservoir into two distinct streams, one liquid and the other gaseous, allows reserves to be exploited by means of conventional technologies which are well known in the petroleum industry. On account of its low density, the gas is easily lifted by means of the small pressure difference between the bottom of the well and the reception vessel located at an onshore processing facility or a production platform, whilst the liquid stream may be lifted, for example, by means of sucker rod pumping (SRP) or another suitable pumping method.
The invention will make it possible to extend to fields with a high gas/liquid ratio, which are restricted to gas lift, the application of artificial lift methods using SRP, progressive cavity pumping (PCP), electrical submersible pumping (ESP) and jet pumping (JP). The gas lift method is inefficient in satellite offshore wells, in onshore wells with long gush lines, in deep wells, in directional (non-vertical) wells and in wells containing viscous oils. As the reservoir becomes depleted, gas lift also becomes less efficient. Many onshore wells are sufficiently depleted that they cannot operate with gas lift, so they operate by using SRP or PCP. These wells, which currently operate inefficiently owing to the low separation efficiency, will benefit from the invention.
In the case of offshore exploitation, separation at the bottom of a well results in a saving of physical space and a reduction in the load on the deck of the production platform.
Greater production may be obtained through the application of the invention, coupled with SRP, PCP or JP, in order to remove condensate in gas wells.
Moreover, in the case of a natural reservoir, a further advantage of this separation process relates to monitoring of reserves. Separate monitoring of the production of liquid and gas will allow better management of the petroleum reservoir. Separation of the liquid and gas flows means that they can be measured more easily, which is important when one considers the difficulties involved in measuring a multi-phase flow.
In addition, in areas other than petroleum production, the invention has an application in industry in general.
The reduction in the efficiency of a petroleum-well pumping system, owing to the presence of free gas, has been known about for some time. The first patent for a separator, for reducing the amount of free gas in the suction region of a bottom pump, was granted in 1881. Since then, many others have been published because, depending on operational conditions, the use of known separators has not always resulted in satisfactory pumping efficiency.
The efficiency of static separators currently in use is low. This is the principal reason for the low volumetric efficiency of sucker rod pumping which, on average, is of the order of fifty per cent. This is a cause for concern, since it is estimated that approximately seventy to eighty per cent of producing wells use sucker rod pumping (SRP), progressive cavity pumping (PCP) or electrical submersible pumping (ESP).
Recently, it has become important to increase the gas-separation efficiency in subsea wells (wet Christmas tree) equipped with electrical submersible pumping (ESP), which is a method applicable in offshore wells equipped with a wet Christmas tree. According to preliminary studies, ESP would appear to be more advantageous than gas lift or underwater multi-phase pumping. Such studies were based on a well-bottom gas-separation efficiency level of the order of ninety per cent. However, it was observed that the efficiency of the available centrifugal separators is not constant, and that it dropped dramatically above a certain flow rate. Principally in the case of offshore wells with high flow rates, the situation is critical since SRP and PCP cannot be used in such wells and ESP requires high separation efficiency which is normally not achieved. This gives rise to a large quantity of gas in the pump which, in turn, increases the number of failures, increases costs and makes centrifugal pumping non-viable.
Amongst currently used bottom separators, the separation efficiency of which is below that which is desired, mention may be made of the following types: natural anchor, conventional (poor boy), cup, packer and inverted shroud. As these are well known, this description will deal, by way of comparison of the separation conditions involving bubbling or cascading, with only the conventional separator.
The process used in known bottom separators normally consists in projecting the two-phase mixture into a medium whose continuous phase is liquid. Under such conditions, the gas is forced to bubble towards the dynamic level of the well, and the efficiency of separation is limited to the speed of ascent of the bubbles in the liquid.
According to Stokes"" Law, the bubbles ascend at a speed which is inversely proportional to the viscosity of the liquid:
v=[g(xcfx81lxe2x88x92xcfx81g)d2]/18xcexcl
in which:
g gravitational acceleration;
xcfx81i liquid density;
xcfx81g gas density;
d bubble diameter;
xcexcl liquid viscosity.
A practical and simplified formula involves a speed of 0.5 (feet per second) divided by the liquid viscosity (centipoise), as proposed by Ryan (1994).
Other authors recommend using Stokes"" Law for Reynolds numbers between 0 and 2, and suggest special equations for other bands.
The present invention proposes the use of a different effect, herein called the xe2x80x9ccascade effectxe2x80x9d, for altering the separation process which has been in use, making the situation similar to what occurs in the case of surface separators.
The cascade separator of this invention, with or without helicoidal surfaces, is installed inside the casing of a well, at the bottom but upstream from the discharge pump, in order to prevent or at least minimize the entry of gas into the pump and consequently to maximize the volumetric efficiency of the pumping operation.
In the equipment of this invention, the two-phase mixture is projected into it, above the liquid level of the separator, into a medium whose continuous phase is gas. Thus, instead of bubbling in a medium in which the continuous phase is liquid, there is a cascade or shower of droplets, whereupon segregation of the gas takes place more rapidly.
However, the conditions of said flow are still not ideal for separation. In order to obtain a more favourable flow, of the xe2x80x9csegregatedxe2x80x9d type, the invention proposes the inclusion of helicoidal surfaces in the descending path of the mixture. The helicoidal surfaces convert the chaotic, descending vertical flow into an inclined, segregated flow, in a free surface channel flow, which better promotes phase separation. On the helicoidal surfaces, the Jukovski""s effect and the thrust caused by the centrifugal acceleration increase the speed of segregation of the bubbles.
U.S. Pat. No. 5,482,117 issued Jan. 9, 1996 describes a helicoidal bottom separator for application in centrifugal pumping. Although helicoidal, that separator is based on a different operating principle from that of this invention. In said patent, the mixture passes over the helicoidal surface in an ascending direction, where it is subjected to the action of centrifugal forces which promote gas separation. The liquid is forced to move to the peripheral part, and the gas to the radially inner part (shaft), of the helicoidal surface. Another important difference is the fact that said separator operates when immersed in liquid, which is the continuous phase, which makes additional segregation of the bubbles problematic. Despite the presence of helicoidal surfaces, a stratified or segregated flow is not achieved. As the movement of the fluid is ascending, a chaotic slugging flow occurs, with the formation of bubbles and a dense mist, which is undesirable for a more efficient separation process.
In the present invention, the descending helicoidal flow is naturally stratified, even in the absence of centrifugal forces, i.e. even if the flow rate or speed of the fluid on the helicoidal surfaces is low. In order to guarantee that gas is the continuous phase, avoiding the formation of slugs or immersion of the helicoidal surfaces, this invention provides:
the installation of a regulating (or controlling) valve in the gas line;
a long separator, in order to contain variations in level, guaranteeing a cascade-type flow;
a perforated separator vessel, in order to allow the entry of the fluid under favourable conditions, separation taking place partly through capillary effect;
a helicoidal surface of variable pitch; and
a gas discharge tube.
U.S. Pat. No. 5,431,228 issued Jul. 11, 1995 is similar to U.S. Pat. No. 5,482,117 discussed above. It is simpler because there is no passage of a drive shaft through its inside. The flow is ascending, presenting the same problems of separation already noted. It may be stated that U.S. Pat. No. 5,482,117 operates principally in wells equipped with electrical submersible pumping and that U.S. Pat. No. 5,431,228 operates in wells equipped with sucker rod pumping, progressive cavity pumping, jet pumping, etc., in which there is no drive shaft passing through the separator.
U.S. Pat. No. 4,981,175 issued Jan. 1, 1991 describes a centrifugal separator in which the helicoidal surfaces rotate whilst the casing remains stationary, there being a clearance between these two components. Because it rotates, the helicoidal surface is known as an impeller or rotor, and requires a motor to actuate it. In the helicoidal separation of this invention, the helicoidal surfaces do not rotate, there is no need for external drive power and the helicoidal surfaces are joined to the casing so that there is no fluid leakage.
U.S. Pat. No. 4,531,584 issued Jul. 30, 1985 is similar to U.S. Pat. No. 5,431,228. Once again, the operating principle is that of ascending helicoidal flow with high speeds so that separation takes place by means of centrifugal effect. This patent, also fails to solve the problems of immersion, which are exacerbated by the existence of tiny, flooded gas passages. The liquid in the annular space floods the radially inner part of the helicoidal surfaces where the gas tends to accumulate. Thus, the conclusion is that it will be difficult for a segregated flow to occur over the helicoidal surfaces and that, over the inner portion thereof, there will be a flow of liquid with a greater concentration of bubbles.
The invention relates to a high-efficiency well-bottom separator, of the xe2x80x9ccascadexe2x80x9d type, which uses helicoidal surfaces to obtain a stratified descending flow, which promotes separation.
More specifically this invention provides a gas separator, for separating out the gaseous phase from a two-phase, liquid/gas mixture, comprising a sedimentation vessel equipped in the upper part with openings for the passage of a production tubing and for the exit of gas has been separated out, and having a lateral surface with an upper portion having through-holes therein; said holes forming, in said lateral surface of the sedimentation vessel, a perforated tube; wherein in use of said gas separator said sedimentation vessel contains liquid, in its lower part, up to a level varying within a selected band below the holes in said perforated tube, and contains predominantly gas in its upper portion, above the level of the separator; and wherein said vessel contains a discharge pump to be connected to receive a production tubing.
Internally, between a production tubing and the inner lateral surface of the sedimentation vessel, over the height of said vessel, there may be helicoidal surfaces. In the upper part of the helicoidal channel there may be a helicoidal discharge tube for part of the gas which has been separated out to flow to the annular space of the well. The lower portion of the separator will be immersed in liquid up to a selected level, which can vary within a certain band, below the perforated portion of the lateral surface.
The invention also relates to the use of such a gas separator at a well bottom.